The present invention relates a method of and an apparatus for painting an object, and more particularly to a method of and an apparatus for painting inner surfaces and outer surfaces such as side and upper surfaces of an automotive vehicle body without paint coat irregularities and efficiently within a small space, so that painted products of high quality can be obtained.
Automobile industry in recent years employs highly automated line production processes for efficiently mass-producing automobile products. To meet desired mass-production requirements, there are used assembling apparatus for assembling individual parts and conveyor apparatus for conveying components to respective working positions. Painting apparatus for painting automobile bodies are also automated.
Various methods have heretofore been employed for automatically painting automobile bodies. According to one known painting method, for example, an automotive vehicle body is conveyed by a conveyor apparatus, and paint spray applied from a plurality of paint spray guns to side panels, an engine hood, a roof, and a trunk lid of the vehicle body as it is conveyed, so that the vehicle body will be painted.
In the above painting method, however, the vehicle body tends to vibrate while it is being conveyed, with the result that a desired paint coating will not be applied accurately but paint coat irregularities will result. Vehicle body painting is one of the critical requirements for determining the quality of completed automobiles. Automobiles with paint coat irregularities or defects cannot be sold in the market. Automotive vehicle bodies with such a defective coating must be painted again to mend the paint defect.
If a vehicle body being conveyed while a paint coat is being applied is positionally displaced as well as vibrated, a paint defect will also arise. Therefore, vehicle bodies must be fixed in position with respect to the conveyor apparatus. However, the procedure for securing the vehicle bodies against vibration and positional displacement is tedious and time consuming.
Another painting method which has been used keeps an automotive vehicle body at rest in a painting position and displaces a painting apparatus with a plurality of paint spray guns with respect to the vehicle body for coating the same. The paint spray guns are arranged in an inverted U-shaped pattern so that they confront the side panels and upper panel of the vehicle body. More specifically, the inverted U-shaped painting apparatus includes two side painting mechanisms located on opposite sides and each having a plurality of paint spray guns, and an upper painting mechanism located on the upper side and having a plurality of paint spray guns directed downwardly. The upper and side painting mechanisms are displaced in unison by a conveyor mechanism with respect to the vehicle body, and a paint coating is applied all over the vehicle body by the paint spray guns.
However, since the upper and side painting mechanisms are moved in unison with respect to the vehicle body, a complex process is required for controlling the upper and side painting mechanisms. More specifically, the upper and side panels of the vehicle body to be painted are of different surface areas, and the upper panel surface lies substantially horizontally whereas the side panel surfaces are inclined considerably with respect to the vertical direction. In order to paint the upper and side panel surfaces appropriately, therefore, the spray guns must be moved at different speeds, and painting conditions such as the rates at which the paint is delivered from the spray guns must be different from painting mechanism to painting mechanism. With the conventional inverted U-shaped painting apparatus, since the upper and side painting mechanisms are moved at the same speed, the coating film on the upper panel surface of the vehicle body tends to be of a small thickness, or paint runs and sags are apt to be produced on the side panel surfaces. Therefore, the rates of discharge of the paint from the spray guns should carefully be controlled.
Where the upper and side painting mechanisms are moved together, the spray guns in the uppermost position on the side painting mechanisms and those at the ends of the upper painting mechanism spray the paint in very close areas, and hence the paint sprays from these spray guns interfere with each other. In electrostating painting where paint is applied under electrostatic forces, the paint particles are electrostatically repelled from each other, resulting in a greater tendency of paint spray interference. Moreover, the paint coat is apt to have different thicknesses which will have to be made uniform by re-painting. Thus, the entire painting procedure is complex, making it difficult to accomplish an efficient automobile production process.